Photonic devices include semiconductor lasers, e.g., vertical cavity surface-emitting lasers (VCSELs) and edge-emitting lasers (EELs), and semiconductor light-emitting diodes (LEDs). Applications for photonic devices are many and include optical communications, optical measuring instruments and optical storage devices.
Photonic devices that generate long-wavelength infra-red light are of great interest in the optical communications industry since existing optical fibers have a relatively low loss in this wavelength range. Wavelengths in the wavelength range that extends from about 1.5 to about 1.6 micrometers (xcexcm), commonly referred to as the 1.55 xcexcm wavelength range, are typically used in optical communications applications, since semiconductor lasers and other components that operate in this wavelength range are relatively low in cost and are widely available. However, optical fibers have a lower optical dispersion in a wavelength range that extends from about 1.25 xcexcm to about 1.35 xcexcm, commonly referred to as the 1.3 xcexcm wavelength range. This wavelength range is less commonly used for optical communications because lasers that operate in this wavelength range are based on an indium phosphide (InP) substrate and so are substantially more expensive that lasers based on a gallium arsenide (GaAs) substrate. Moreover, it is difficult to make VCSELS that operate in the 1.3 xcexcm wavelength range due to the lack of suitable mirror materials compatible with InP.
The active layer of a photonic device is the layer in which electrons and holes combine to generate light. Although it is possible to make photonic devices with a homogeneous active layer, an active layer that includes a quantum-well structure provides the photonic device with a lower threshold current, a higher efficiency and a greater flexibility in choice of emission wavelength.
A quantum-well structure is composed of at least one (n) quantum-well layer interleaved with a corresponding number (n+1) of barrier layers. Each of the quantum well layers has a thickness in the range from about one nanometer to about ten nanometers. The barrier layers are typically thicker than the quantum well layers. The semiconductor materials of the layers of the quantum-well structure depend on the desired emission wavelength of the photonic device. The semiconductor material of the barrier layers differs from that of the quantum-well layer, and has a larger bandgap energy and a lower refractive index than that of the quantum well layer.
The active layer is composed of the quantum-well structure sandwiched between two cladding layers. The semiconductor materials constituting the quantum-well structure are typically undoped. One of the cladding layers is doped n-type, the other of the cladding layers is doped p-type. Thus, the active layer has a p-i-n structure.
A quantum-well structure composed of gallium arsenide antimonide (GaAsSb) quantum-well layers with gallium arsenide (GaAs) barrier layers has been proposed for the active region of VCSELS structured to generate light with a wavelength of 1.3 xcexcm. FIG. 1 is an energy-band diagram of an exemplary active layer 10 incorporating such a quantum-well structure having one quantum-well layer. Band energy is plotted as ordinate and distance from the substrate is plotted as abscissa.
The active layer 10 is composed of the substrate-side cladding layer 12, the substrate-side barrier layer 14 of GaAs, the quantum-well layer 16 of GaAsSb, the remote-side barrier layer 18 of GaAs and the remote-side cladding layer 20. The energy-band diagram of FIG. 1 shows the energies of the conduction band 22 and the valence band 24 of the semiconductor material of each of the layers just described.
The quantum-well structure composed of the barrier layers 14 and 18 of GaAs and the quantum-well layer 16 of GaAsSb has what is known as a Type II heterostructure. In a Type II heterostructure, the energy of the valance band 24 of the GaAsSb of the quantum-well layer 16 is greater than the energy of the valance band of the GaAs of the barrier layers 14 and 18 and the energy of the conduction band 22 of the GaAsSb of the quantum-well layer is also greater than the energy of the conduction band of the GaAs of the barrier layers.
The line-up of the band energies in a quantum-well structure having a Type II heterostructure confines electrons 26 to the conduction band 22 of the barrier layers 14 and 18 and confines holes 28 to the valance band 24 of the quantum-well layer 16. As a result, the electron-hole recombination process occurs between carriers confined in physically-different layers and is called spatially indirect. An active layer incorporating a quantum-well structure having a Type-II heterostructure can emit and absorb photons with energies well below the bandgap energy of the material of either the quantum-well layer or the barrier layers. Photonic devices incorporating such an active layer operate at wavelengths much longer than those corresponding to the bandgap energies of the materials of the quantum-well structure. However, active layers incorporating a quantum-well structure having a Type-II heterostructure have a relatively low gain due to the low overlap between the electron and hole wave functions.
Another disadvantage of active layers incorporating a quantum-well structure having a Type II heterostructure is that edge-emitting lasers incorporating such an active layer have a threshold current density that depends on the device dimensions and an operating wavelength that depends on the operating current. These variations in threshold current density and operating wavelength can lead to problems in lasers used in optical communications applications where channel spacings of a few hundred GHz impose strict wavelength stability requirements.
In addition, for the active region to generate light at 1.3 xcexcm, the GaAsSb of the quantum-well layer 16 has an antimony (Sb) fraction of about 0.35, i.e., x=xcx9c0.35 in GaAs1-xSbx. With this antimony fraction, the GaAsSb has a lattice constant substantially larger than that of GaAs, so that the quantum-well layer is under substantial compressive strain when grown on GaAs. It is therefore difficult to fabricate active regions having more than one or two quantum wells without an unacceptably high defect density occurring as a result of relaxation of the strain. Barrier layers of GaAs are incapable of providing strain compensation for quantum-well layers of GaAsSb grown on a substrate of GaAs.
An alternative quantum-well structure that has been proposed for the active region of VCSELs structured to generate light at 1.3 xcexcm is composed of gallium arsenide antimonide (GaAsSb) quantum-well layers with aluminum gallium arsenide (AlGaAs) barrier layers. FIG. 2 is an energy-band diagram of an exemplary active layer 40 incorporating such a quantum-well structure having one quantum-well layer. As in the energy-band diagram of FIG. 1, band energy is plotted as ordinate and distance from the substrate is plotted as abscissa.
The active layer 40 is composed of the substrate-side cladding layer 42, the substrate-side barrier layer 44 of AlGaAs, the quantum-well layer 46 of GaAsSb, the remote-side barrier layer 48 of AlGaAs and the remote-side cladding layer 50. The energy-band diagram shows the energies of the conduction band 22 and the valence band 24 of the semiconductor materials of the layers just described.
The active layer composed of the barrier layers 44 and 48 of AlGaAs and the quantum-well layer 46 of GaAsSb has what is known as a Type I heterostructure. In a Type I heterostructure composed of GaAsSb and AlGaAs, the energy of the valance band 24 of the GaAsSb of the quantum-well layer 46 is greater than the energy of the valance band of the AlGaAs of the barrier layers 44 and 48, but the energy of the conduction band 22 of the GaAsSb of the quantum-well layer is less than the energy of the conduction band of the AlGaAs of the barrier layers.
The line-up of the band energies in a quantum-well structure having a Type I heterostructure confines electrons 56 to the conduction band 22 of the quantum-well layer 46 and confines holes 58 to the valance band 24 of the quantum-well layer 46. As a result, the electron-hole recombination process takes place between carriers confined in the same layer, a recombination process called spatially direct, and the gain of the active region 40 can be substantially higher than that of the active region 10 shown in FIG. 1. Additionally, the threshold current density and operating wavelength of photonic devices incorporating active regions with a quantum-well structure having a Type I heterostructure have little dependence on the device dimensions and operating current, respectively.
However, the high reactivity of the aluminum in the AlGaAs of the barrier layers 44 and 48 makes the active layer 40 difficult to fabricate with good crystalline quality, high optical quality and high operational reliability consistent with the growth requirements of GaAsSb.
Moreover, for the active region to generate light at 1.3 xcexcm, the GaAsSb of the quantum-well layer 46 has an antimony (Sb) fraction of about 0.35, i.e., x=xcx9c0.35 in GaAs1-xSbx. It is desirable that the quantum-well structure provide an electron confinement of at least 4-5 kT (where k is Boltzmann""s constant and T is the temperature in Kelvin) to reduce carrier leakage over the heterojunction energy barriers at room temperature. Providing the desired electron confinement with an Sb fraction of about 0.35 in the GaAsSb of the quantum well layer 46 requires an Al fraction of about 0.25 or more, i.e., zxe2x89xa70.25 in AlzGa1-zAs, in the AlGaAs of the barrier layers 44 and 48.
Antimony has a low equilibrium vapor pressure over GaAsSb, and GaAsSb has a low melting point and, hence, a low thermodynamic stability temperature. Consequently, the GaAsSb quantum-well layer 46 has to be grown at a growth temperature about 100xc2x0 C. lower than the growth temperatures at which AlGaAs and GaAs are conventionally grown. Moreover, the thermodynamic stability of the Gaxe2x80x94As bond is low and the Sb species has a low volatility. Consequently, the GaAsSb quantum-well layer is also epitaxially grown with a very low V/III ratio and a low As/Ga ratio. The low As over-pressure and the low growth temperatures are detrimental to the crystalline quality of the AlGaAs barrier layers. Under these conditions, the high reactivity of the aluminum of the barrier layers 44 and 48 results in the AlGaAs of the barrier layers incorporating carbon and oxygen from the MOCVD precursors. These impurities act as a non-radiative recombination centers and additionally impair the long-term reliability of the photonic device. The carbon incorporated in the AlGaAs of the barrier layers 44 and 48 increases the background p-type doping level and leads to high free carrier absorption. This results in increased optical losses and a correspondingly increased threshold current density.
Finally, since AlGaAs has substantially the same lattice constant as GaAs, barrier layers of AlGaAs lack the ability to provide strain compensation between the GaAsSb of the quantum-well layer and the GaAs of the substrate. This limits the number of quantum-well layers that can be included in the quantum-well structure without an unacceptably high density of defects.
Thus, what is needed is an active region for long-wavelength photonic devices that provides such photonic devices with a low threshold current, a stable operating wavelength and a high quantum efficiency. In particular, what is needed is an active region having a Type I heterostructure in combination with a high gain, a high optical transparency, a low free carrier loss and a low density of non-radiative recombination centers. The materials constituting the active layer should have conduction and valence band offsets large enough to prevent carrier leakage under high current drive conditions. The active region should be capable of generating light in the 1.3 xcexcm wavelength range and be substantially lattice matched to a GaAs substrate.
The invention provides a long-wavelength photonic device that comprises an active region that includes at least one quantum-well layer of a quantum-well layer material that comprises InyGa1-yAsSb in which yxe2x89xa70, and that additionally includes a corresponding number of barrier layers each of a barrier layer material that includes gallium and phosphorus. The barrier layer material has a conduction-band energy level greater than the conduction-band energy level of the quantum-well layer material and has a valence-band energy level less than the valence-band energy level of the quantum-well layer material.
As a result of the relationships between the band energy levels, the active layer includes a Type I heterostructure in which both holes and electrons are confined in the quantum-well layer and the hole-electron recombinations are spatially direct. As a result, the photonic device has a high gain, a high quantum efficiency and a low threshold current. The photonic device generates long-wavelength light at a wavelength that is substantially independent of temperature and operating current. Moreover, a barrier layer material containing phosphorus can be grown with a high V/III ratio without the risk of damaging the surface of the quantum-well layer, thus ensuring sharp, well-defined interfaces between the quantum-well layer and the barrier layers. Finally, barrier layer materials that include In and P are capable of providing strain compensation between the quantum-well layers and substrates of GaAs or InP. This enables quantum-well structures that include multiple quantum-well layers to have a low density of defects.
Other systems, methods, features, and advantages of the present invention will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims.